Process for producing nanocrystalline alpha-AI2O3

ABSTRACT

A process is claimed for producing nanocrystalline α-Al 2 O 3  that is characterized in that nuclei are added to aluminum chlorohydrate, which is subjected to thermal treatment within less than 30 minutes, the resultant agglomerates being comminuted.

The present invention relates to a process for producing nanocrystallineα-Al₂O₃, also referred to as corundum, very finely dispersecrystallization nuclei, preferably α-Al₂O₃ nuclei, first being added toaluminum chlorohydrate in the form of a solution as starting material,which is then subjected to a thermal or thermophysical reaction.

Ultrafine alumina powders are used in particular for ceramicapplications, for matrix reinforcement of organic or metallic layers, asfillers, as polishing powders, for the production of abrasives, asadditives in paints and laminates and for further special applications.The production of the ultrafine alumina powders is effected either bychemical synthesis, mechanical comminution methods or a thermophysicalroute.

The chemical synthesis generally involves precipitation reactions(hydroxide precipitation, hydrolysis of organometallic compounds) withsubsequent calcination. Frequently, crystallization nuclei are added inorder to reduce the transformation temperature to α-alumina. The solsthus obtained are dried by an inconvenient procedure and thus convertedinto a gel. The further calcination then takes place at temperatures offrom 350° C. to 650° C. For the transformation to α-Al₂O₃, ignition mustthen be effected at temperatures of about 1000° C. The processes aredescribed in detail in DE 199 22 492.

A further route for obtaining nanomaterials is the aerosol process.There, the desired molecules are obtained from chemical reactions of aprecursor gas or by rapid cooling of a supersaturated gas. The formationof the particles is effected either by collision or the constantvaporization and condensation of molecular clusters in equilibrium. Thenewly formed particles grow through further collision with productmolecules (condensation) and/or particles (coagulation). However, if thecoagulation rate is greater than that of new formation or of growth,agglomerates of spherical primary particles form.

Flame reactors represent a production variant based on this principle.Here, nanoparticles are formed by the decomposition of precursormolecules in the flame at 1500° C.-2500° C. The oxidations of TiCl₄,SiCl₄ and Si₂O(CH₃)₆ in methane/O₂ flames may be mentioned as examplesand lead to TiO₂ and SiO2 particles. With the use of AlCl₃, it has beenpossible to date to produce only the corresponding alumina. Flamereactors are used today industrially for the synthesis ofsubmicroparticles, such as carbon black, pigment TiO₂, silica andalumina. It has not been possible to date to produce nanoscale corundumin this way.

Small particles can also be formed with the aid of centrifugal force,compressed air, sound, ultrasound and further methods, also from drops.The drops are then converted into powder by direct pyrolysis or by insitu reactions with other gases. Spray-drying and freeze-drying may bementioned as known processes. In spray pyrolysis, precursor drops aretransported through a high temperature field (flame, oven), which leadsto rapid vaporization of the readily volatile component or initiates thedecomposition reaction to give the desired product. The desiredparticles are collected in filters. The preparation of BaTiO₃ from anaqueous solution of barium acetate and titanium lactate may be mentionedas an example here.

By milling, it is also possible to attempt to comminute corundum and toproduce crystallites in the nano range thereby. The best milling resultscan be achieved with stirred ball mills in wet milling. Milling beadscomprising a material which has a greater hardness than corundum must beused. However, this method is characterized by high wear of millingbeads, contamination of the product with the material of the millingbeads, an enormous time requirement and a very high energy consumption.

A further route for the production of corundum at low temperature is thetransformation of aluminum chlorohydrate. Nuclei, preferably comprisingvery fine corundum or hematite, are likewise added thereto for thispurpose. For avoiding crystal growth, the samples must be calcined attemperatures of about 700° C. to not more than 900° C. The duration ofthe calcination here is at least 4 hours. A disadvantage of this methodis therefore the great time requirement and the residual amounts ofchlorine in the alumina. The method was described in detail in reportDKG 74 (1997) No. 11/12, pages 719-722.

The disadvantages of the processes according to the known prior art arethat the yields per unit time are low owing to the long calcinationtimes or, in the case of milling, the product is contaminated and isstill too coarse.

The object of the present invention is therefore to producenanocrystalline corundum by a process which gives high yields in a shorttime with minimum energy supply. The product produced should beredispersible by simple means and hence capable of giving stablenanosuspensions.

Contrary to the statements by various authors to date (report DKG 74(1997) No. 11/12; DE 199 22 492), this object can be achieved startingfrom aluminum chlorohydrate (aluminum hydroxychloride).

Surprisingly, it was found that nanocrystalline corundum can be producedwithin a few minutes on calcination of aluminum chlorohydrate.

The invention relates to a process for producing nanocrystallineα-Al₂O₃, nuclei being added to aluminum chlorohydrate and the latterbeing subjected to thermal treatment, and the agglomerates thus obtainedbeing comminuted. The duration of the thermal treatment is less than 30minutes.

The starting point for the process according to the invention isaluminum chlorohydrate, which is attributed the formulaAl₂(OH)_(x)Cl_(y), where x is a number from 2.5 to 5.5 and y a numberfrom 3.5 to 0.5 and the sum of x and y is always 6. This aluminumchlorohydrate is mixed as aqueous solution with crystallization nuclei,then dried and then subjected to a thermal treatment (calcination).

50% strength aqueous solutions of aluminum chlorohydrate, as arecommercially available, are preferably used as starting material.Crystallization nuclei which promote the formation of the α-modificationof Al₂O₃ are added to such a solution. In particular, such nuclei reducethe temperature for the formation of the α-modification during thesubsequent thermal treatment. Suitable nuclei are very finely dispersecorundum, diaspore or hematite. Very finely disperse α-Al₂O₃ nucleihaving a mean particle size of less than 0.1 μm are preferably employed.In general, from 2 to 3% by weight of nuclei, based on the resultingalumina, are sufficient.

This suspension of aluminum chlorohydrate and nuclei is then evaporatedto dryness and subjected to a thermal treatment (calcination). Thiscalcination is effected in apparatuses suitable for this purpose, forexample in sliding-bat furnaces, chamber furnaces, tube furnaces, rotarykilns or microwave ovens or-in a fluidized bed reactor. According to onevariant of the process according to the invention, it is also possibleto adopt a procedure in which the aqueous suspension comprising aluminumchlorohydrate and nuclei is injected directly into the calcinationapparatus without prior removal of the water.

The temperature for the calcination should not exceed 1100° C. The lowertemperature limit is dependent on the desired yield of nanocrystallinecorundum, on the desired residual chlorine content and on the content ofnuclei. The corundum formation begins at as low as about 500° C. but, inorder to keep the chlorine content low and the yield of nanocrystallinecorundum high, it is preferable to work at from 700 to 1100° C., inparticular at from 1000 to 1100° C.

The time for the calcination is in general less than 30 minutes,preferably from 0.5 to 10, in particular from 0.5 to 5, minutes. Evenafter this short time, a sufficient yield of nanocrystalline corundumcan be achieved under the abovementioned conditions for the preferredtemperatures.

During the calcination, agglomerates of nanocrystalline corundum areobtained in the form of virtually spherical primary crystallites, theterm “nanocrystalline” being understood as meaning a particle size of ingeneral from 20 to 100 nm. For obtaining these primary particles, theagglomerates are comminuted by wet or dry milling, preferably by wetmilling in water, for example in an attritor mill, air-jet mill orstirred ball mill. These agglomerates are deagglomerated in a subsequentstep, it being possible to use all deagglomeration methods known inceramics, since in the present case the agglomerates are ones which canbe relatively easily destroyed. Wet or dry milling is preferably carriedout for the deagglomeration, the wet milling preferably being effectedin an attritor while the dry milling is carried out in an air-jet mill.Since the nanoparticles strived for as a product in the milling areextremely reactive, additives which prevent reagglomeration of thenanoparticles are preferably added before or during the milling. It istherefore particularly advantageous to carry out the subsequentdeagglomeration in the form of wet milling. Vibration mills, attritors,ball mills, stirred ball mills or similar apparatuses are suitable forthe wet milling. The use of stirred ball mills has been found to beparticularly advantageous. The duration of milling depends on thestrength of the agglomerates themselves and, in the process according tothe invention, is usually from 2 to 6 hours. The wet milling ordeagglomeration is advantageously carried out in an aqueous medium butit is also possible to use alcoholic or other organic solvents. Thus,for example after milling for two hours in water, an aqueous suspensionof nanocrystalline corundum having a d50 value of less than 100 nm isobtained. During the wet milling, it is also advantageous additionallyto use additives which become attached to the surface of the particlesand prevent agglomeration even during a subsequent drying step. Suitablematerials for this purpose are polyvinyl alcohols, stearates, waxemulsions, acrylates or polyethylene glycols. The suspension obtainedafter the wet milling can be converted into a defined powder byspray-drying, fluidized-bed drying or granulation.

The process according to the invention makes it possible, starting froma commercially available raw material, to produce nanocrystallinecorundum in a very much shorter time compared with the processesaccording to the prior art.

The nanocrystalline corundum thus produced can be used in a multiplicityof applications, for example for the production of ceramic or ofabrasives.

EXAMPLES Example 1

2% of crystallization nuclei of a suspension of very fine corundum wereadded to a 50% strength aqueous solution of aluminum chlorohydrate.

After the solution was homogenized by stirring, drying is effected in arotary evaporator. The solid aluminum chlorohydrate was comminuted in amortar, a coarse powder forming.

The powder was calcined in a muffle furnace at 1050° C. The contact timein the hot zone was not more than 5 min. A white powder whose particledistribution corresponded to the feed material was obtained.

An X-ray structure analysis shows that it is a single-phase α-alumina.

The images of the SEM (scanning electron micrograph) produced showedcrystallites in the range 10-100 nm. The residual chlorine content wasonly a few ppm.

Example 2

The solid aluminum chlorohydrate with 2% crystallization nuclei fromexample 1 was fed to a laboratory rotary kiln. In this example, too,single-phase α-alumina forms in a contact time of not more than 5minutes and at a temperature of 1050° C.

The other properties of the powder obtained are identical to the productfrom example 1.

Example 3

150 g of corundum powder from example 1 were suspended in 150 g ofwater. The suspension was fed to a vertical stirred ball mill fromNetzsch (type PE 075). The milling beads used consisted of zirconiumoxide (stabilized with yttrium) and had a size of 0.3-0.5 mm.

The particle distribution of the feed material was 20-100 μm. Afterrespectively one, two, three and four hours, the mill was stopped and asample was taken. In addition, the pH was checked at these times. The pHwhich increases with progressive deagglomeration was kept at pH 5 byaddition of dilute hydrochloric acid. The samples taken hourly werecharacterized by means of a Horiba particle sizer.

It was found that the primary crystallites had agglomerated onlyslightly after 4 h. The coarse primary particles (20-100 μm) were almostcompletely destroyed after only 1 h, so that a d50 of 270 nm wasdetectable after this time. The d50 after 4 h was <108 nm.

Example 4

For this example, an acrylate polymer (Dispex® A40, Ciba) was used forstabilizing the nanoparticles, instead of hydrochloric acid.

150 g of corundum powder from example 1 were suspended in 150 g ofwater. In addition, 1 % of Dispex A40, based on the mass of corundum,was mixed with the suspension. The suspension was fed to a verticalstirred ball mill from Netzsch (type PE 075). The milling beads usedconsisted of zirconium oxide (stabilized with yttrium) and had a size of0.3 -0.5 mm.

The particle distribution of the feed material was 20-100 μm. Afterrespectively one, two, three and four hours, the mill was stopped and,depending on the viscosity of the suspension, further Dispex A40 wasadded so that the final content was about 4%. The suspension had a d50of 148 nm after 4 h.

Example 5

The wax emulsion Licowax® KST (Clariant) was added to the suspensionfrom example 3 with stirring, the proportion of wax, based on thecorundum, being about 3%. The dispersion thus obtained was spray-driedin a laboratory dryer from Büchi (Mini Spray Dryer B-290). The air inlettemperature was 180° C. and the temperature of the exit air was 100° C.A loose spray powder which can be used for processing comprising shapingin the area of ceramics and abrasives was obtained.

1. A process for producing nanocrystalline Al₂O₃, comprising the stepsof adding a crystallization nuclei to aluminum chlorohydrate, thermallytreating the combined crystallization nuclei and aluminum chlorohydratefor less than 30 minutes to form agglomerates, and comminuting theagglomerates.
 2. The process as claimed in claim 1, wherein the aluminumchlorohydrate is a compound having the chemical formulaAl₂(OH)_(x)Cl_(y), wherein x is a number from 2.5 to 5.5 and y is anumber from 3.5 to 0.5, and wherein the sum x+y is always
 6. 3. Theprocess as claimed in claim 1, wherein the crystallization nuclei isvery finely dispersed α-Al₂O₃, hematite or diaspore.
 4. The process asclaimed in claim 3, wherein the very finely dispersed α-Al₂O₃crystallization nuclei has a mean particle size of less than 0.1 μm. 5.The process as claimed in claim 1, wherein an aqueous suspensioncomprising aluminum chlorohydrate plus crystallization nuclei is firstdried and the dried product is then calcined.
 6. The process as claimedin claim 1, wherein the thermally treating step is carried out in asliding-bat furnace, chamber furnace, tube furnace, rotary kiln,microwave oven or in a fluidized-bed reactor.
 7. The process as claimedin claim 1, wherein the thermally treating step is carried out attemperatures below 1110° C.
 8. The process as claimed in claim 1,wherein the thermally treating step is carried out at from 700 to 1110°C.
 9. The process as claimed in claim 1, wherein an aqueous suspensioncomprising aluminum chlorohydrate and nuclei is injected directly intothe calcination apparatus without prior removal of the water.
 10. Theprocess as claimed in claim 1, wherein the thermally treating step iscarried out in from 0.5 to 30 minutes.
 11. The process as claimed inclaim 1, wherein the agglomerates are comminuted by wet or dry milling.12. The process as claimed in claim 1, wherein the agglomerates arecomminuted by wet milling to form a suspension, and wherein at least oneadditive selected from the group consisting of acrylates, polyvinylalcohols, polyethylene glycols, stearates and wax emulsions is addedduring or after the wet milling.
 13. The process as claimed in claim 1,wherein the agglomerates are comminuted by wet milling to form asuspension, and the suspension is spray-dryed.
 14. The process asclaimed in claim 1, wherein the thermally treating step is carried outin from 0.5 to 10 minutes.
 15. The process as claimed in claim 1,wherein the thermally treating step is carried out in from 2 to 5minutes.